The determination of chlorpheniramine at low part per billion levels in blood serum is being developed to support a clinical study to be conducted under the direction of Paul Turkeltaub, M.D., Allergenic Products laboratory, DBP, CBER. One procedure, that has been evaluated and partially validated, involves extraction, concentration and gas chromatography. Chlorpheniramine is detected by selective ion monitoring mass spectrometry of a characteristic fragment ion at m/e 203. Extraction efficiency and injection volume are normalized by addition of tetradeuterated chlorpheniramine and monitoring of the analogous D4 fragment ion at m/e 207. Development of an alternate method, that would permit the resolution of chlorpheniramine into its enantiomeric components, is in progress. Enantiomeric resolution has not been obtained using gas chromatographic chiral columns. HPLC columns based on alpha-glycoprotein and beta-cyclodextrin stationary phases have provided enantiomeric resolution, but detection of chlorpheniramine at ppb levels has not been accomplished by UV absorbance. Other HPLC detection methods have not been evaluated as providing necessary sensitivity. Work is in progress on a pre-column derivatization method involving reaction with benzylchloroformate, chiral HPLC and fluorescence detection that may provide the necessary sensitivity for detection of chlorpheniramine at serum equilibrium levels while allowing enantiomeric speciation.